Sunan an-Nasa'i 1241
سنن النسائي#1241Sahihأَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ الْمُبَارَكِ الْمُخَرَّمِيُّ، قَالَ حَدَّثَنَا وَكِيعٌ، عَنْ مِسْعَرٍ، عَنْ مَنْصُورٍ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيَتَحَرَّ وَيَسْجُدْ سَجْدَتَيْنِ بَعْدَ مَا يَفْرُغُ " .
It was narrated that 'Abdullah said:"The Messenger of Allah (ﷺ) said: 'If one of you is not sure about his prayer, let him estimate and prostrate twice after he has finished
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 13:63